A wide-area positioning system, such as the Global Positioning System (GPS), uses a constellation of satellites to position or navigate objects on earth. Each satellite in the GPS system currently transmits two carrier signals, L1 and L2, with frequencies of 1.5754 GHz and 1.2276 GHz, and wavelengths of 0.1903 m and 0.2442 m, respectively. Next generation Global Navigation Satellite Systems (GNSS), such as the modernized GPS and Galileo systems, will offer a third carrier signal: L5. In the GPS system, L5 will have a frequency of 1.1765 GHz, and a wavelength of 0.2548 m.
Two types of GPS measurements are usually made by a GPS receiver: pseudorange measurements and carrier phase measurements.
The pseudorange measurement (or code measurement) is a basic GPS observable that all types of GPS receivers can make. It utilizes the C/A or P codes modulated onto the carrier signals. With the GPS measurements available, the range or distance between a GPS receiver and each of a plurality of satellites is calculated by multiplying a signal's travel time (from the satellite to the receiver) by the speed of light. These ranges are usually referred to as pseudoranges because the GPS measurements may include errors due to various error factors, such as satellite clock timing error, ephemeris error, ionospheric and tropospheric refraction effects, receiver tracking noise and multipath error, etc. To eliminate or reduce these errors, differential operations are used in many GPS applications. Differential GPS (DGPS) operations typically involve a base reference GPS receiver, a user GPS receiver, and a communication mechanism between the user and reference receivers. The reference receiver is placed at a known location and is used to generate corrections associated with some or all of the above error factors. Corrections generated at the reference station, or raw data measured at the reference station, or corrections generated by a third party (e.g., a computer or server) based on information received from the reference station (and possibly other reference stations as well) are supplied to the user receiver, which then uses the corrections or raw data to appropriately correct its computed position.
The carrier phase measurement is obtained by integrating a reconstructed carrier of the signal as it arrives at the receiver. Because of an unknown number of whole cycles in transit between the satellite and the receiver when the receiver starts tracking the carrier phase of the signal, there is a whole-cycle ambiguity in the carrier phase measurement. This whole-cycle ambiguity must be resolved in order to achieve high accuracy in the carrier phase measurement. Whole-cycle ambiguities are also known as “integer ambiguities,” after they have been resolved, and as “floating ambiguities” prior to their resolution. Differential operations using carrier phase measurements are often referred to as real-time kinematic (RTK) positioning/navigation operations.
High precision GPS RTK positioning has been widely used for many surveying and navigation applications on land, at sea and in the air. The distance from the user receiver to the nearest reference receiver may range from a few kilometers to hundreds of kilometers. As the receiver separation (i.e., the distance between a reference receiver and a mobile receiver whose position is being determined) increases, the problem of accounting for distance-dependent biases grows and, as a consequence, reliable ambiguity resolution becomes an even greater challenge. The major challenge is that the residual biases or errors after double-differencing can only be neglected for ambiguity resolution purposes when the distance between the two receivers is less than about 10 km. For longer distances the distance-dependent errors, such as orbital error and ionospheric and tropospheric delays, become significant problems. Determining how long the observation span should be to obtain reliable ambiguity resolution is a challenge for GPS RTK positioning. The longer the observation span that is required, the longer the “dead” time during which precise positioning is not possible. The ambiguity resolution process is required at the start of GPS navigation and/or surveying and whenever to many of the GPS signals are blocked or attenuated such that cycle slips or measurement interruptions occur. Quality control of the GPS RTK positioning is critical and is necessary during all processes: data collection, data processing and data transmission. Quality control procedures are applied to both the carrier phase-based GPS RTK positioning and to the pseudo-range-based DGPS. The quality control and validation criterion for ambiguity resolution represents a significant challenge to precise GPS RTK positioning.